Theoretical And Experimental Study On Novel Hollow-core Waveguides Based On Hyperbolic Metamaterials And Bandgap Structures

Hollow-core waveguide,due to its air-core structure,provides several optical properties different from conventional solid-core waveguides,becoming a very unique category of passive optical waveguides.Hollow-core waveguides have sparkled novel applications in high-capacity high-speed communication systems,sensing systems,mid-infrared/Terahertz(THz)systems and high-power laser delivery.The refractive index of the air-core is 1,while the dielectrics in nature are generally greater than 1 and the losses on metal waveguide surface are extremely high.As a result,the guiding mechanism of hollow-core is different from that of all-solid waveguide(total internal reflection).In order to confine and guide the electromagnetic wave in a low-index core,the waveguide cladding needs to be specifically designed,basically exploiting new materials or structures.This Thesis is devoted to theoretically study,simulate and experimentally characterize hollow-core waveguides with metal wires based hyperbolic metamaterial and photonic bandgap structures in the cladding,achieving single-mode manipulation of THz radiations and orbital angular momentum(OAM)propagations at THz frequency as well as optical communication band.Major innovative results of the Thesis are summarized as follows.1.We have proposed a hollow-core waveguide with sub-wavelength scale metallic wires in the cladding.The theoretical studies show that such a hybrid cladding reflects the transverse magnetic(TM)waves and transmits the transverse electric(TE)waves,leading to a waveguide structure that only confines TM modes in a wavelength-scale air-core.Compared to conventional metallic waveguides,the single-mode operating window of hyperbolic metamaterial waveguide is twice;while compared to common dielectric waveguides,which are usually multi-mode,the hyperbolic metamaterial waveguide has a size-reduced core.The numerical simulation results show that,only a ring of metal wires are required in the cladding to well confine TM modes,which greatly simplify the waveguide manufacturing process.2.According to the unusual optical characteristics of metal wires based hyperbolic metamaterial,we have studied,fabricated,and experimentally characterized a wide-band,single-mode,radial-polarization hollow-core THz waveguide.From numerical simulations,the proposed waveguide offers a 2.3 times wider single-mode bandwidth compared to dielectric-coated metallic waveguides with the same core dimension.Within the single-mode window,the propagation loss can be as low as 0.28 dB/cm.From experimental characterization using THz time-domain-spectroscopy,we observe single-mode guidance with radial-polarization between 0.31 and 0.44 THz in a wavelength-scale air core(diameter of 0.88 mm).The transmission spectrum and modal imaging illustrate the characteristic that the hybrid cladding reflects TM waves,which verifies the theoretical prediction.3.Symmetrically replacing several metal wires with air-holes,we have proposed a linearly polarized single-mode hollow-core THz waveguide with a hybrid cladding.The optimized waveguide provides a single linear polarization single-mode operating window covering 0.36-0.46 THz with propagation losses of?0.25 dB/cm,and the loss ratios to other modes are around 10-2.Thanks to the excellent confinement of TM modes in a wavelength-scale core(diameter of 1mm),the proposed waveguide offers low bend losses(<0.3 dB/cm at 0.4 THz)for bend radii larger than 5 cm.The low-loss linearly polarized TM mode could be readily excited by a Gaussian beam with coupling efficiency up to 67%,which is more than twice of that of radially polarized mode.4.We have characterized the OAM transmission in hollow-core THz waveguides with numerical simulations.The high order modes as well as OAM modes in Kagome hollow-core THz waveguide are analyzed in detail.Simulation results show that,two anti-resonant transmission windows can be found over 0.2-0.9 THz,where three OAM modes can be generated.The purity of OAM modes in the second band(0.6-0.85 THz)are higher than 0.9.Comparing Kagome hollow-core THz fiber with different ring numbers of Kagome structures,we reveal the confinement performance affects the OAM purity.Moreover,the OAM propagation performances in inner-dielectric coating metallic THz waveguide and Bragg hollow-core THz waveguide are also investigated.The drawbacks of those two THz waveguides for OAM applications are presented.5.We have discussed the viability of exploiting hollow-core photonic bandgap fibers(HC-PBGFs)to support orbital angular momentum(OAM)states at optical communication band.The high order vector modes and corresponding OAM modes in HC-PBGFs with different core sizes are simulated using numerical models.For 7-cell HC-PBGF,a broad transmission window with satisfied effective index separations between vector modes(no less than 10-4)and low confinement loss(below 3 dB/km)for the first order OAM mode covering 240 nm bandwidth is observed.The OAM purity could reach 0.952 at 1710 nm.In 19-and 37-cell HC-PBGFs that have larger core dimensions,the propagation losses reduce,OAM mode number and purity values increase,yet the anti-crosstalk ability becomes worse.The influences of non-circular symmetry of fiber core and effective mode area on OAM purity are analyzed.Furthermore,we also present the tolerance of OAM propagation to fabrication inaccuracies.